Reduction of carboxylic acids to primary alcohols using LiAlH4

by James

Description: Addition of lithium aluminum hydride [LiAlH4] to carboxylic acids leads to the formation of primary alcohols (after addition of acid)
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What’s probably leaving here is the aluminum salt, Li(+) (-)O-AlH2 . You could draw a resonance form as Li(+) O=AlH2 (-) ; the strong Al-O bond stabilizes the negative charge that will be formed here. I should probably adjust the drawing, thanks.

Most of the reactions I’ve done in lab I don’t do an acidic workup. This tends to produce low yields. It is best to use a NaOH(aq) soln between 15%-25% this causes much of the salts to precipitate out and you can almost just pour off your organic layer and have some close to pure product. This is pretty standard via Fieser and Fieser for metal hydride workups…
For every mole of carboxylic acid use 3-5 molar equivalents of LiAlH4. Typically run with a THF solvent under inert atmosphere/ dry conditions, in which the carboxylic acid soln is added dropwise to the LiAlH4 suspended in THF at 0*C. Workup is for every Xg of LiAlH4 add XmL of H2O slowly, XmL of 15-25% NaOH(aq) soln, 3XmL of H2O, then allowing to warm to room temperature and stirred for variable amounts of time, then separated, precipitated salts washed , and combined organic layer and extractions dried with MgSO4, filtered and your done. Through it in a column and you get even analytically pure product.
Anyways I personally love the detailed mechanism, but could you maybe do one for a basic workup as well. I’m finding organic mechanisms and reactions in my textbooks are not the whole story or are not conducive to what really goes on in lab.

You are absolutely correct. The Fieser workup that you mention is the most practical way to work up this reaction. “Acid” could refer here to water, which serves as a proton source for the aluminum hydride.

I had to make a choice when starting this site: cater to students just starting out, or to aim matters at a more advanced type of reader. Due to the relatively scattered resources available for students in the first category, I chose the former.

You are clearly in the second category. There are more resources available for someone in your situation, such as March’s Advanced Organic chemistry, Not Voodoo, Chemical Forums, Reich’s site at Wisconsin, and a lot of other blogs and websites.

I wish I could easily please both groups of people but I had to make a choice. Thanks for understanding. James

Yeah I use Not Voodoo all the time it is an amazingly helpful website. I’m personally having trouble with purification/isolation of my product (nicotinic acid –> 3-pyridinemethanol) due to functional groups that like to make Zwitterion’s which is why I asked. So I’ll definitively have to check out these other sources. Thanks alot!